The raw materials for the production of asphalt, such as virgin aggregate or recycled asphalt pavement (RAP), are typically loaded from outdoor stockpiles into a plurality of individual hoppers and fed therefrom to a conveyor and then into a drum mixer. The drum mixer removes moisture and heats the aggregate. The heated aggregate may be coated with liquid asphalt either in the mixer or in a separate mixing chamber or pugmill.
In a typical mixer, wet aggregate enters the drum and is picked up and dropped through the hot gases and gradually cascades from one end of the drum to the other. The dwell time for material in the drum typically varies from as short as two minutes to as long as six minutes. The raw materials must absorb a relatively large amount of heat from the drum mixer for a satisfactory final product.
Since the moisture content of the raw materials affects the amount of heat that must be absorbed in the drum mixer, an effort is typically made to manage the stockpiles of raw materials to reduce moisture. Raw materials may be taken from the sections of the outdoor stockpiles which have benefitted most from drainage and natural heating and drying. For example, the moisture level for a typical aggregate material varies according to height above the ground surface with the material adjacent the ground having the highest moisture content. Therefore, a loader operator may be instructed to remove material from above the 3' level, for example. In addition, the stockpile may be positioned on a slight incline and the loader operator may also be instructed to remove material from the sunny side of the stockpile. Stockpiles may also be covered to reduce their moisture content. As disclosed in Productivity: "Producing Profits for the '80s", by J. Don Brock, Technical Bulletin No. T-106, 1984, it has been suggested that rising fuel prices may make it desirable to heat stockpiles with forced hot air generated by a water heat exchange system coupled to the exhaust gases from the baghouse of a typical asphalt plant.
Heating of raw materials is used in many different processes of a typical asphalt plant. For example, U.S. Pat. No. 4,172,445 to Sellers discloses a heater provided for a storage tank for the storage of fluids, such as asphalt or hot water. The oil fired burner is fitted into a flue container of a ceramic combustion chamber which has a thick wall and serves to shield the outside of the flue from overheating the asphalt for the length of the ceramic chamber. Similarly, U.S. Pat. No. 3,757,745 to Miller discloses dual extending heat exchangers positioned within a storage tank for asphalt or other liquids.
U.S. Pat. No. 3,659,583 to Martin is directed to maintaining the aggregate material in a storage bin free from a light freeze or frost condition. Martin discloses passing heated air through jackets on the walls of the bins to heat the aggregate only to an increment above the freezing point of water. The Martin patent teaches that it is not feasible to pass steam through the aggregate since the condensed steam thoroughly soaks the mass. On the other hand, if dry, flue gases from a heat generator are passed through the aggregate, contamination may result which is equally as unacceptable as steam soaking.
U.S. Pat. No. 4,387,996 to Mendenhall discloses a batch plant using the Minnesota Heat Transfer Method of Recycling, in which a small amount of preheat is given to the RAP in a separate drum where the RAP is in direct contact with the combustion gases in order to reduce the amount of super heat required for mixing with the virgin aggregate material in the drum mixer.